Pulsed coherent laser radar is capable of remote measurements of atmospheric winds and turbulences over ranges of several kilometers. A pulsed coherent laser radar can also be used for shorter range applications where high degrees of accuracy and integrity are required. This invention relates to such short range application in general and the measurement of aircraft air data in particular.
A coherent laser radar as an aircraft air data sensor uses the aerosols in the atmosphere to scatter some of the transmitted energy from the laser source back to a receiver where the Doppler shift is used to measure the aircraft velocity component along the transmitter line of sight. By pointing the laser beam in three or more directions, the aircraft true speed and angles of attack and sideslip can be measured. Compared with continuous wave laser radar, a pulsed system can provide more accurate and reliable measurements. This is due to the enhanced return signal from high peak power pulses.
A pulsed coherent laser radar based on the conventional Injection-Seeded Oscillator/Amplifier (ISOA) technique uses a highly stable low power continuous wave laser as master oscillator and a Q-switched pulse laser as slave laser. This laser system utilizes a feedback control system for injection locking of the two laser frequencies. The feedback control consists of a detector for monitoring the slave laser spectral characteristics, electronic circuitry generating control signals, and a piezoelectric translator for adjusting the slave laser resonator length. In this technique, part of the master oscillator is used as an optical local oscillator and the remaining part for injection seeding of the slave laser. The output of the slave laser is then transmitted through a telescope into the atmosphere.
Recently, a pulsed coherent laser radar was developed that used a single laser source to generate both the transmitter and local oscillator beams. This technique utilizes the self-seeding technique to generate single frequency pulses to be transmitted. By actively controlling the laser Q-switched transmission, a low power quasi-continuous wave pulse is generated that trails the Q-switched pulse. The trailing portion is separated from the Q-switched pulse and is used as the local oscillator.
The technique described in this disclosure also utilizes the self-seeding technique for generating single frequency pulses. However, the additional feedback control loop for generation of the trailing quasi-continuous wave beam has been eliminated to further simplify the system design. This is achieved by utilizing an optical modulator device to modulate the intensity of a portion of the laser pulse generating a flat top pulse suitable as a local oscillator. This technique is best suited for short range applications, since the local oscillator flat top pulse width needs to be equal to or longer than the transmitter round trip time to the desired range.
An object of this invention is to present a method and apparatus for interferometric laser measurement of short range atmospheric conditions.
A second object of this invention is to present a method and apparatus for single frequency laser interferometric measurement of short range atmospheric conditions, e.g., as applied to aircraft true airspeed, angle of attack and angle of sideslip.
A further object of this invention is to present a pulsed coherent laser radar as an aircraft air data sensor which uses the aerosols in the atmosphere to scatter some of the transmitted energy from the laser source back to a receiver where the Doppler shift is used to measure the aircraft velocity component along the transmitter line of sight